A honeycomb structure into which a catalyst is loaded is conventionally used for purification treatment of a toxic substance, such as HC, CO, or NOx, included in exhaust gas emitted from an engine of an automobile or the like. When the catalyst loaded into the honeycomb structure treats the exhaust gas in this manner, it is required to raise a temperature of the catalyst up to the active temperature of the catalyst. However, when the engine starts, there is a problem that the exhaust gas is insufficiently purified because the catalyst has not reached the active temperature. Particularly, since the traveling by a plug-in hybrid vehicle (PHEV) or a hybrid vehicle (HV) includes traveling by only a motor, the frequency of starting the engine is low and the temperature of the catalyst is low when the engine starts. As a result, the performance of purifying the exhaust gas immediately after the engine starts easily degrades.
In order to solve the above problem, there has been proposed an electrically heated catalyst (EHC) in which an electrode is disposed in a honeycomb structure formed by conductive ceramics and then energization causes the honeycomb structure itself to generate heat to make it possible to raise a temperature up to an active temperature before an engine starts (see Patent Document 1).
There has been proposed another heating device in which a tubular resistance heating type heater is arranged so as to surround a circumferential wall of a honeycomb structure and heats a wall surface of the circumferential wall of the honeycomb structure to transmit the heat to an inner cell structure (see Patent Document 2).